Acoustic ink printing has been identified as a promising direct marking technology. See, for example, Elrod et al. U.S. Pat. No. 4,751,530 on "Acoustic Lens Array for Ink Printing", Elrod et al. U.S. Pat. No. 4,751,529 on "Microlenses for Acoustic Printing", and Elrod et al. U.S. Pat. No. 4,751,534. on "Planarized Printheads for Acoustic Printing". The technology is still in its infancy, but it may become an important alternative to ink jet printing because it avoids the nozzles and small ejection orifices that have caused many of the reliability and pixel placement accuracy problems which conventional drop on demand and continuous stream ink jet printers have experienced.
This invention builds upon prior acoustic ink printing proposals relating to the use of focused acoustic radiation for ejecting individual droplets of ink on demand from a free ink surface at a sufficient velocity to deposit them in an image configuration on a nearby recording medium. Droplet ejectors embodying acoustic focusing lenses, such as described in the aforementioned Elrod et al patents, and piezoelectric shell transducers, such as described in Lovelady et al U.S. Pat. No. 4,308,547, which issued Dec. 29, 1981 on a "Liquid Drop Emitter," have been proposed for carrying out such printing. Moreover, techniques have been developed for modulating the radiation pressure which such beams exert against the free ink surface, thereby permitting the radiation pressure of any selected beam to make brief, controlled excursions to a sufficiently high pressure level for ejecting individual droplets of ink from the free ink surface (i.e., a pressure level sufficient to overcome the restraining force of surface tension) on demand.
As is known, acoustic ink printers of the foregoing type are sensitive to variations in their free ink surface levels. Even if the half wave resonances of their resonant acoustic cavities are effectively suppressed as taught by an Elrod et al U.S. patent application, which was filed Dec. 21, 1988 under Ser. No. 07/287791 for "Acoustic Ink Printers Having Reduced Focusing Sensitivity", the size and the velocity of the ink droplets they eject are difficult to control, unless their free ink surfaces remain within the effective depth of focus of their droplet ejector or ejectors. Preferably, therefore, the free ink surface level of such a printer is closely controlled. For instance, the depth of focus of state of the art acoustic lens type droplet ejectors typically is comparable to the wavelength of the acoustic radiation in the ink.
To that end, prior acoustic ink printers have included provision for maintaining their free ink surfaces at more or less constant levels. For example, a copending and commonly assigned Elrod et al. U.S. patent application, which was filed on Dec. 19, 1986 on "Variable Spot Size Acoustic Printing" suggests using a closed loop servo system for increasing and decreasing the level of the free ink surface under the control of an error signal which is produced by comparing the output voltage levels from the upper and lower halves of a split photodetector. The magnitude and sense of that error signal are correlated with the free ink surface level because a laser beam is reflected off the free ink surface to symmetrically or asymmetrically illuminate the opposed halves of the photodetector depending upon whether the free ink surface is at a predetermined level or not. As will be appreciated, that sometimes is a workable solution to the problem, but it is costly to implement and requires that provision be made for maintaining the laser and the split photodetector in precise optical alignment. Moreover, it is not well suited for use with larger droplet ejector arrays because the surface tension of the ink tends to cause the level of the free ink surface to vary materially when the free surface spans a large area.
Ink transport mechanisms also have been proposed for refreshing the ink supplies of such printers, including transports having apertures for entraining the ink while it is being transported from a remote inking station to a position in acoustic alignment with the printhead. See Quate U.S. Pat. No. 4,801,953, which issued Jan. 31, 1989 on "Perforated Ink Transports for Acoustic Ink Printing". Also see Quate U.S. Pat. No. 4,797,693, which issued Jan. 10, 1989 on "Polychromatic Acoustic Ink Printing". However, the free ink surface level control that is provided by these transports is dependent upon the uniformity of the remote inking process and upon the dynamic uniformity of the ink transport process.